In the manufacture or cartons for milk, juices and other liquids in common sizes of quarts, 2-liter and half-gallons, it is common practice to cut blanks of the carton from a sheet of multi-layered, laminated, paperboard which includes a thermoplastic polymeric material, such as polyethylene on its exterior surfaces. These blanks include cut edges which are free of the thermoplastic. The blanks commonly are scored along fold lines to define multiple panels, and subsequently erected into a four-sided tubular geometry. This formed tube is thereafter fitted onto a mandrel having a flat outboard end. While on this mandrel, the bottom-forming panels of the blank are infolded toward the longitudinal centerline of the tube to define a closed bottom of the tube. The infolded bottom panels are captured between the flat end of the mandrel and a pressure pad disposed externally of the tube. Application of heat to the panels and pressure via the pressure pad, effects heat sealing of the bottom-defining panels to one another to complete the bottom.
It will be noted from FIG. 1 herein that each of the outer perimetral edges of a blank is a cut edge. Each such edge is free of thermoplastic material and therefore is susceptible to absorption and wicking of liquid into the interior structure of the paperboard with consequential weakening of the carton wall or bottom.
A typical carton bottom includes at least first and second bottom panels disposed opposite one another across the bottom of the carton and which are designed to be infolded toward the longitudinal centerline of the tubular carton. This infolding defines two multiple-layered generally triangular segments of the bottom, the apices of the triangular segments being disposed adjacent one another at or near the longitudinal centerline of the carton. Others of the bottom panels are of generally rectangular geometry and are infolded toward the centerline of the carton to position their outboard side edges transverse of the carton bottom and lying generally at or adjacent one of the transverse centerlines of the plane of the carton bottom. Each of these other bottom panels commonly comprises a single ply or layer of the paperboard, but as the panels are folded inwardly to define the bottom of the carton, multiple layers of paperboard are developed in certain, but not all, areas of the bottom of the carton. Moreover, (a) the number of layers may vary between one and five layers, for example, (b) the location of the those areas which include more than one layer of paperboard frequently are not contiguous over the bottom of the carton, and (c) the actual area of the bottom which is defined by a given number of layers varies in size (e.g., two or more areas of four layers thickness may be at different locations over the area of the bottom. These and other factors militate against the desired uniform application of that sealing pressure and heat distribution over the area of the bottom of the carton which will result in full effective sealing of the bottom panels. In the prior art, it has been the practice to provide a patterned pressure pad which includes a pattern of raised areas on that surface of the pad which engages the infolded bottom panels. For each folding configuration of the bottom panels of a paperboard carton, heretofore, there has been provided a specifically patterned pad. These prior art pressure pads are basically designed to develop inordinately large pressure in selected regions of the carton bottom in an attempt to over compensate the pressure aspect of the heat sealing operation, the theory being that higher pressure in a given area will ensure a good seal in such area. The result of these prior art pressure pads is deleterious compression of the thickness of the paperboard in these high pressure areas. In use of the carton, these "thin walled" areas of the paperboard tend to develop leaks in the carton, especially the carton bottom.
Each fold in a bottom panel presents a possibility of leakage of liquid into or from the carton. Heat sealing of the bottom panel along the folds thereof is complicated by the relative bulkiness of the panels at these folds. Caution must be exercised in squeezing of the folds in that excessive squeezing of a fold between the pressure pad and the mandrel can create weakened areas in the folds which can be the source of leakage of liquid into or from the carton in the course of its life from formation through consumer use.
Still further, the presence of portions of the cut edges of the blank within the bottom of the carton are particularly susceptible to liquid absorption and wicking. In an effort to reduce the overall exposed area of one or more of the cut side edges of the bottom panels and to permit these side edges to more effectively be sealed within the bottom of the carton, these cut side edges may be skived, then folded back upon themselves and bonded together in an effort to eliminate exposure of a cut edge of the blank to liquid and elimination or reduction of wicking of liquid at the cut edge of the blank. The skived and back-folded side edges of a panel present a still further thickness value (ie, a thickness of more or less than one layer of the paperboard) to be dealt with in the formation of the bottom of the carton. In addition to the consideration of the thickness of the skived and back-folded panel edges, consideration must be given to the width of the back-fold to ensure full sealing of this side edge against the wicking of absorbed liquid along the length dimension of the side edge if liquid happens to bypass other sealing locations associated with the side edge.
All of these militating factors associated with the heat sealing of the bottom panels of a carton bottom are made more problematic by the industry practice of designing different geometric configurations of the bottom panels of the carton. As will be recognized, each different combination of geometric configurations of the bottom panels brings about different heat sealing requirements for each given combination of bottom panels. In one known instance, as many as twenty different bottom panel combinations exist for forming the bottom of a paperboard tubular carton for different liquid contents of the carton. As a consequence, each pressure pad must be especially designed for use with a specific combination of paperboard and folding pattern for a given carton. Thus, when it is desired to change from one bottom folding design to another design, it has been necessary that the carton-forming apparatus be taken off-line and the pressure pads thereof changed. Commonly, carton-forming apparatus includes a plurality of mandrels mounted on a carousel so that many pressure pads must be changed each time there is a switch between bottom folding designs. In addition to the time and expense of this pad changeout procedure, it is required that the manufacturer maintain an inventory of each type of pressure pad, at substantial cost to the manufacturer.
For many years, the industry has sought a pressure pad which is universal with respect to the number of different carton bottom geometrical configurations which can be successfully sealed with the pad. This search has been partially confounded by the further need for locating, i.e., alignment of, the infolded bottom panels of the intended carton bottom with respect to the pressure pad. Misalignment of the pad relative to the infolded panels of the carton bottom can result in each seal being misaligned with respect to the infolded panels. Such misalignment can result in complete or partial failure of seal formation and/or in deleterious thinning out of the paperboard at various locations over the area of the bottom of the carton, such as in the folds, and resultant premature failure of the bottom of the carton in use. In the prior art, as each pressure pad was designed to accommodate a variety of carton bottom designs, the need for proper alignment increased, thereby discouraging the use of such pads.
Further, alignment (ie., spacing apart) of the exposed sculpted surface of the pressure pad relative to the flat surface of a mandrel disposed internally of the tubular carton and in opposition to the pressure pad, is also required to ensure proper separation distance between the mandrel surface and the exposed surface of the pressure pad. Otherwise, there may be more or less of the required pressure applied to the panels of the carton bottom which are captured between the mandrel and pressure pad, with the result being possible rupture of the folds of the panels, incomplete sealing of the panels to one another, and/or the application of excessive pressure in some regions of the bottom, and insufficient pressure in other regions of the bottom. In this respect it is to be noted that known carton-making machines which employ a mandrel fitted inside the tubular carton for forming the closed bottom of the carton, include a pressurized piston-cylinder device mounted in alignment with the mandrel. This cylinder has the pressure pad mounted on the end of the piston for movement of the piston toward and away from the mandrel. Adjustment of the position of the cylinder and its piston and pad mounted thereon relative to the mandrel is provided for so as to permit adjustment of the position of the pressure pad relative to the mandrel.
In one common design for the bottom of a paperboard carton for liquids, a portion of the .sup.5 th panel, integrally bonded to a side edge of one of the bottom panels, extends into the bottom of the carton. This design creates unusual sealing problems so that alignment of the pad and mandrel in this area of the carton bottom has been used for aligning the pad and mandrel. Heretofore, it has been the practice to attempt a first trial alignment of the pad and carton bottom, perform a few trial sealing operations, and then visually observe whether the "stake" seals associated with the 5.sup.th panel are properly aligned with the 5.sup.th panel. If misalignment was observed, the entire alignment process was repeated until proper alignment was achieved.